A cage & roller type of needle roller bearing composed of rollers and a retainer is employed as an idler bearing of a car transmission, and a bearing for a connecting rod large end of a motorcycle engine in many cases. Such bearing is disclosed in Japanese Unexamined Patent Publication No. 2000-257638 (Patent Document 1), for example.
According to this document, a pipe-shaped material is formed into an annular member having an M-shaped cross section by a bulging process, and pockets to hold rollers are formed in the annular member, whereby a roller bearing retainer light in weight and large in load capacity can be provided.
The roller bearing retainer having the above constitution is guided on the outer diameter side in which a column end part and a housing are in contact with each other at the time of rotation, in many cases. Here, when a surface area of an outer diameter surface of the column end part being in contact with the housing is small, the contact part could be abraded and burned.
In addition, the pocket is formed between adjacent column parts. The column part is composed of a column center part, the column end part, and a column sloped part. A circumferential interval between the adjacent column center parts or the adjacent column end parts is set to be smaller than a roller diameter of a roller, and a circumferential interval between the adjacent column sloped parts is set to be larger than the roller diameter of the roller. Thus, it is reported that the roller can be effectively prevented from dropping out of the retainer without providing a projection at a wall surface of the column part opposed to the pocket.
When the roller bearing retainer is formed by the method disclosed in the above document, the thickness of bend parts such as a boundary part between the column center part and the column sloped part, a boundary part between the column sloped part and the column end part, and a boundary part between the column end part and an annular side part is thinner than that of the pipe-shaped material. Since stress applied to the retainer at the time of the rotation of the bearing concentrates on the bend part, the roller bearing retainer could be easily damaged because the bend part is thinned.
In addition, when the roller bearing retainer is formed by the method disclosed in the above document, since the curvature radius of the bend parts such as the boundary part between the column center part and the column sloped part, the boundary part between the column sloped part and the column end part, and the boundary part between the column end part and the annular side part is larger than that of the retainer, the length of a linear part of the column end part cannot be longer.
In addition, according to the roller bearing retainer disclosed in the above document, when the roller has a small diameter, it is difficult to prevent the roller from dropping out. Meanwhile, when the circumferential interval between the adjacent column parts is reduced in order to prevent the roller from dropping out of the retainer for sure, the play amount of the roller cannot be sufficiently provided.